1. Field of the Invention
The invention relates to an MR apparatus for imaging of an examination zone, including a magnet for generating in the examination zone a uniform, steady magnetic field having substantially parallel lines of force extending in a first direction, a gradient coil system for generating a magnetic gradient field whose gradient can be varied in respect of magnitude and/or direction, an RF coil system for generating RF pulses and for receiving MR signals, means for generating data from the MR signals, a reconstruction unit for reconstructing an MR image of the examination zone from a set of the data, patient support means comprising a table top and drive means for displacement of the table top in said first direction, and means for coupling said drive means and said control unit.
2. Description of the Related Art
An example of an MR apparatus of this kind is disclosed in U.S. Pat. No. 5,498,961, incorporated herein by reference. This document describes an MR method for two-dimensional or three-dimensional imaging of the examination zone, said method being now generally known as helical scanning. In this method, in order to produce an image representing the examination zone at a selectable instant or in a selectable slice, a set of auxiliary data is formed from at least two sets of raw data, the auxiliary data being derived by interpolation from the raw data acquired with the same measurement parameters, but at different measurement instants, or derived for different measurement slices. The weight applied to the raw data entering the interpolation is greater as the distance in time between the associated measurement instant and the selectable instant, or between the measurement slice and the selected slice, is smaller. The image of the examination zone is reconstructed from the set of auxiliary data.
Consequently, in the helical scanning method the images of the examination zone are not reconstructed directly from the raw data, but from the auxiliary data derived from the raw data by interpolation. This interpolation takes into account the instants of measurement of the raw data (or the position of the measurement slice) as well as the selectable instant (or the position of the selected layer) for which an image of the examination zone is to be reconstructed. The temporal resolution can then be higher than the value corresponding to the measurement period required for acquisition of a set of raw data. The same holds for the spatial resolution in the direction of movement of a moving object.
Due to these features, the helical scanning method is suitable for examining an object which is moved relative to the examination zone during the examination. In that case, sets of raw data are continuously measured during the movement, at least one set of auxiliary data being formed from said raw data, a respective MR image being reconstructed from said auxiliary data. This enables the formation of a sequence of cross-sectional images of the body already during the introduction of the (patient's) body into the examination zone, the spatial resolution in the movement direction already having been improved. The apparatus referred to above is particularly suitable for carrying out this version of the helical scanning method.